1. Field of the Invention
The present invention generally relates to an electromagnetic servo device. More particularly, the invention relates to an electromagnetic servo device of a type suitable for use in an electric type power steering system for vehicles.
2. Description of Relevant Art
In view of problems on the hydraulic type power steering system such as that the structure thereof was complicated, recent years have proposed a variety of electric type power steering systems for vehicles. In those electric type power steering systems were employed various types of electromagnetic servo device.
Exemplarily, in Japanese Patent Application No. 59-192390, filed on Sept. 12, 1984, and its corresponding commonly assigned U.S. Pat. application Ser. No. 773,972 filed Sept. 5, 1985; there has been proposed by the present applicant an electromagnetic servo device for electric power steering systems for vehicles.
This electromagnetic servo device included an input shaft adapted to be operatively connected to a steering wheel, an output shaft adapted to be operatively connected to a road wheel to be turned, a torque detection mechanism for detecting the magnitude and the direction of torque acting on the input shaft, and a driving control circuit adapted to produce, based on a set of output signals from the torque detection mechanism, a torque magnitude signal and a torque direction signal and to feed, according to these signals, an electric motor, which was provided for supplying auxiliary torque to the output shaft, with an armature current of a necessary quantity, in a necessary direction of conduction. The electric motor was coaxially fitted around the output shaft, to be rotatable relative thereto, and adapted to have torque electromagnetically developed therein to be transmitted to the output shaft, through a reduction gearing and an electromagnetic clutch both fitted likewise around the output shaft.
The driving control circuit had generated, besides the armature current fed to the electric motor, which current was of the necessary quantity that was proportional to the torque magnitude signal, a drive signal for driving the electromagnetic clutch, the drive signal being obtained by overlapping the torque magnitude signal on the torque direction signal.
In other words, in this electromagnetic servo device, the electric motor was fed with an armature current in proportion to the magnitude of steering torque acting on the input shaft, and the electromagnetic clutch was driven to function with a clutching force in correspondence to the magnitude of the steering torque. Accordingly, with increase in the magnitude of the steering torque acting on the input shaft, the armature current to be fed the electric motor became large and also the clutching force of the electromagnetic clutch, that is, the function of torque transmission thereof, was intensified.
Incidentally, in this servo device, the torque magnitude signal, the torque direction signal, and the clutch drive signal had dead zones thereof with predetermined widths, respectively.
In this electromagnetic servo device, the torque developed at the electric motor was transmitted as auxiliary torque through the reduction gearing and the electromagnetic clutch in a smooth manner to the output shaft, whereby, for a driver of the vehicle, the burden that otherwise he or she had to bear when operating the steering wheel was adequately reduced, thus providing favorable steering characteristics.
However, such electromagnetic servo device had, in a driving control circuit thereof, a motor driving system and a clutch driving system respectively constituted as independent open loops relative to each other. There was thus found the desideratum to maintain a well matching state of clutching force with respect to motor torque.
In this respect, if the matching had been well controlled, such electromagnetic servo device could have rendered the steering characteristic of electric power steering system more favorable.
The present invention has been achieved to accomplish such desideratum in a conventional electromagnetic servo device.